To say that the atmosphere begins at Earth’s surface and extends upward is obvious. However, where does the atmosphere end, and where does outer space begin? There is no sharp boundary; the atmosphere rapidly thins as you travel away from Earth, until there are too few gas molecules to detect.

Pressure Changes

To understand the vertical extent of the atmosphere, let us examine the changes in atmospheric pressure with height. Atmospheric pressure is simply the weight of the air above. At sea level, the average pressure is slightly more than 1000 millibars (mb). This corresponds to a weight of slightly more than 1 kilogram per square centimeter (14.7 pounds per square inch). The pressure at higher altitudes is less ▼.

One-half of the atmosphere lies below an altitude of 5.6 kilometers (3.5 miles). At about 16 kilometers (10 miles), 90 percent of the atmosphere has been traversed, and above 100 kilometers (62 miles), only 0.00003 percent of all the gases making up the atmosphere remains. Even so, traces of our atmosphere extend far beyond this altitude, gradually merging with the emptiness of space.

Temperature Changes

By the early twentieth century, much had been learned about the lower atmosphere. The upper atmosphere was partly known from indirect methods. Data from balloons and kites showed that near Earth’s surface, air temperature drops with increasing height. This phenomenon is felt by anyone who has climbed a high mountain and is obvious in pictures of snow-capped mountaintops rising above snow-free lowlands. We divide the atmosphere vertically into four layers on the basis of temperature ▼.

Troposphere

The lowermost atmospheric layer, in which we live, where temperature decreases with an increase in altitude, is the troposphere. The term literally means the region where air “turns over,” a reference to the appreciable vertical mixing of air in this lowermost zone. The troposphere is the chief focus of meteorologists because it is in this layer that essentially all-important weather phenomena occur.

The temperature decrease in the troposphere is called the environmental lapse rate. Although its average value is 6.5°C per kilometer (3.5°F per 1000 feet), a figure known as the normal lapse rate, its value is variable. To determine the actual environmental lapse rate as well as gather information about vertical changes in pressure, wind, and humidity, radiosondes are used. A radiosonde is an instrument package that is attached to a balloon and transmits data by radio as it ascends through the atmosphere ▼.

The thickness of the troposphere is not the same everywhere; it varies with latitude and season. On average, the temperature drop continues to a height of about 12 kilometers (7.4 miles). The outer boundary of the troposphere is the tropopause.

Stratosphere

Below the tropopause, atmospheric properties, such as temperature and humidity, are readily transferred by large-scale turbulence and mixing. Above the tropopause, in the stratosphere, there is very little mixing and turbulence. Temperatures increase in the stratosphere because it is in this layer that the atmosphere’s ozone is concentrated. Recall that ozone absorbs ultraviolet radiation from the Sun. As a consequence, the stratosphere is heated. In the stratosphere, the temperature remains constant to a height of about 20 kilometers (12 miles) and then begins a gradual increase that continues until the stratopause, at a height of nearly 50 kilometers (30 miles) above Earth’s surface.

Mesosphere

In the next highest atmospheric layer, the mesosphere, temperatures again decrease with height until, at the mesopause—approximately  kilometers ( miles) above the surface—the temperature approaches -90°C (-130°F). The coldest temperatures anywhere in the atmosphere occur at the mesopause. Because accessibility is difficult, the mesosphere is one of the least explored regions of the atmosphere. It cannot be reached by the highest research balloons, nor is it accessible to the lowest orbiting satellites. Recent technological developments, such as satellite monitoring, sounding rockets, and ground-based measurements, are just beginning to fill this knowledge gap.

Thermosphere

The uppermost atmospheric layer extends outward from the mesopause and has no well-defined upper limit. It is the thermosphere, a layer that contains only a tiny fraction of the atmosphere’s mass. In the extremely thin air of this outermost layer, temperatures again increase, due to the absorption of very short-wave, high-energy solar radiation by atoms of oxygen and nitrogen.

Temperatures rise to extremely high levels of more than 1000°C (1800°F) in the thermosphere. But such temperatures are not comparable to those experienced near Earth’s surface. Temperature is defined in terms of the average speed at which molecules move. Because the gases of the thermosphere are moving at very high speeds, the temperature is very high. But the gases are so sparse that, collectively, they possess only an insignificant quantity of heat. For this reason, the temperature of a satellite orbiting Earth in the thermosphere is determined chiefly by the amount of solar radiation it absorbs and not by the high temperature of the almost nonexistent surrounding air. If an astronaut inside were to expose his or her hand, the atmosphere would not feel hot.

• Because the atmosphere gradually thins with increasing altitude, it has no sharp upper boundary but simply blends into outer space.

• Based on temperature, the atmosphere is divided vertically into four layers. The troposphere is the lowermost layer. In the troposphere, temperature usually decreases with increasing altitude. This environmental lapse rate is variable but averages about 6.5°C per kilometer (3.5°F per 1000 feet). Essentially, all important weather phenomena occur in the troposphere.

• Beyond the troposphere is the stratosphere, which warms with increasing altitude because of absorption of UV radiation by ozone. In the mesosphere, temperatures again decrease with increasing altitude. Above the mesosphere is the thermosphere, a layer with only a tiny fraction of the atmosphere’s mass and no well-defined upper limit.

• environmental lapse rate: The rate of temperature decrease with increasing height in the troposphere.

• mesosphere: The layer of the atmosphere immediately above the stratosphere and characterized by decreasing temperatures with height.

• radiosonde: A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.

• stratosphere: The layer of the atmosphere immediately above the troposphere, characterized by increasing temperatures with height, due to the concentration of ozone.

• thermosphere: The region of the atmosphere immediately above the mesosphere and characterized by increasing temperatures due to absorption of very shortwave solar energy by oxygen.

• troposphere: The lowermost layer of the atmosphere. It is generally characterized by a decrease in temperature with height.